There are many important safety concerns with medical fluid systems. One important safety concern in connection with medical fluid systems, especially those transferring drugs or fluids to and from a patient, is the need to ensure that the medication or fluid is traveling to the appropriate place at the appropriate time. A number of conditions can arise to disrupt the proper flow of fluid within a medical fluid system. For instance, even if the system is otherwise operating properly, the lines to and from the pump may become kinked or otherwise obstructed, for example, due to the patient rolling over onto a line.
Another concern with fluid flow systems, especially larger systems, is to ensure that its myriad of valves are working properly, e.g., are not leaking and are not blocked. Larger systems, such as the System 1000® hemodialysis/hemofiltration machine produced by the assignee of the present invention, include many valves, some of which are pivotal to the operation and safety of the machine. For instance, there are valves that operate with the fluid pumps that either direct fluid from a source to the patient (e.g., to the patient's blood line) or from the patient to drain. Also, in hemodialysis and hemofiltration operations, a net amount of fluid called ultrafiltrate (“UF”) is removed from the patient over the course of therapy. Certain valves play a critical role in operating with a UF metering device to ensure that the correct amount of fluid is taken from the patient. In any of those situations, a faltering valve can have adverse effects.
To ensure that fluid is flowing in medical fluid systems, flow sensors are employed. The flow sensors operate with a system controller to ensure, in critical situations, that fluid flow is present in a line when it supposed to be present and is not present when, for example, a valve is supposed to be closed. The flow sensor sends a signal indicating whether there is flow or no flow and in certain cases how much flow. The controller compares the signal outcome to an expected outcome and either allows the system to continue functioning if the actual outcome matches the expected outcome or commands an evasive action or alarm to occur if the actual outcome is different than the expected outcome.
Known fluid flow sensors have included a temperature sensitive device positioned in the fluid flow path, wherein the device is connected in a circuit. The temperature sensitive device, such as a thermistor, responds to temperature changes associated with fluid flow changes to produce a change in the circuit. The circuit change signals a certain response, for instance, an alarm to the controller. Temperature based flow sensors have had certain historical disadvantages due to their poor sensitivity to temperature changes, slow response time, fragility (the sensors typically contain a glass bead) and difficulty in being manufactured. Those disadvantages in certain instances have compromised the reliability of the fluid systems employing those sensors.
Accordingly, an improved flow sensing apparatus for medical fluid systems is needed.